Oil Pressure, myths, facts and just plain bollocks
Aug 14, 2020 9:27:13 GMT
primerstag and ott like this
Post by richardthestag on Aug 14, 2020 9:27:13 GMT
While Carl is sweeping up the remains of SOCForum after what would appear to have been yet another hacking attack 
I know that you know it Kryten
and Flying Farmer but it is also worth mentioning to less knowledgeable folk that too much oil pressure could cause as much damage as too little. Oil flow is more important in almost every case.
And before you all start blowing steam out of your ears/arse/whatever, on my first engine I was fighting what I believed to be perpetual falling oil pressure. Cold start was invariably 45psi but this dropped to 20psi at hot idle, 2k + was 30-35psi. Full on panic mode ensued!
this was 25 years ago
I replaced the pump release valve which raised it a bit at hot idle but did nowt for the 2k+ rev range, by now terrified
I replaced the pump with a saab jobby which gave be 70psi+ from cold, but the oil pressure dropped to much the same when hot and at idle
BUT
A little knowledge here would have informed me of some of the behaviours displayed on the mechanical oil pressure gauge. i.e. coasting the oil pressure was ok, 55-60psi iirc, however hard acceleration and it dropped by upto 10psi
. this situation continued to get worse over the following 6 months. I tried to keep it below 3k rpm at all times
I believe that the main bearings were worn to such an extent that the 20/50 was squirting out the sides and dripping back into the sump, this was depriving other parts of the engine of oil, lower oil pressure may have avoided this situation and prevented the destruction of the block.
This happened suddenly, disassembly helped me to understand better what had happened.
Main bearings were all worn, centre was the worst by some margin. #2 and #4 were back to the copper. I believe that the crank vibration caused 2,3 and,4 main bearing caps to fracture. The engine ran but with a noticeable death knock from cold (this only happened once, and the last time that the engine ran) and oil pressure was no higher that 20psi. and barely 5psi at idle.
Not unusual for a Rover v8 to run at 10psi or less at idle, my first 3.9 Range Rover happily sat with the oil pressure light flickering at idle, far from ideal but it ran. Before you all start thumping the table and shouting "but what about the hydraulic chain tensioners?" Well I am not sure what pressure they need to run at but personally I never had rattling chains on the engine that died so can only assume that even when idle was recording below 10psi and the oil light was flickering the chains had enough oil pressure!! Kryten / flying farmer what are your feelings on chain tensioners and pressure required?
Rover v8 hydraulic tappets will work fine at very low pressure indeed
, I know that they are slightly different in that they are primed and tend to be very slow to leak back but this 3.9 v8 had done 180k miles so was hardly in the prime of yoof.
Now we all know about how wonderful hindsight is, so what would I do if faced with the same issue again?
Well first of all I wouldn't have fitted an oil pressure gauge without at least first understanding the characteristics and behaviours that it might display. After all without this all the gauge is tell you is that the gauge is working! Milothedog taught we that one
More importantly I think is to understand the design characteristics of the engine, what parameters was it designed to run under? Having had the luxury of building a few Stag v8 engines up from scratch and running them as if new I am no longer terrified if the cold start oil pressure is not reading 55psi, mine has been just below 50psi since I rebuilt it a few years back. it idles hot at around 30psi and runs somewhere around 40-45psi. Nothing ever changes, the consistency of it's behaviour is almost boring. I may as well remove the oil pressure gauge now because it should be used as a diagnostic device rather than something to distract while on the road.
So to you folk out there sobbing into your cornflakes because your stag v8 is running at whatever psi at idle? Rest thy sphincters. Your engine may well be past youth and it doesn't necessarily spell disaster. Key thing if you have an oil pressure gauge is less about what it displays and more about sudden changes in normal everyday behaviour. And if it does! try another gauge, preferably mechanical before loosing all hope and diving into the pit of despair.
Now that original engine of mine, was the unit fitted to the car when new
, the car had just under 100k miles on the clock. The cylinders were standard bore, needed a rebore but were serviceable. The crank journals were also standard, vandervell bearings and everything. The heads were unskimmed. Just wish that I had the opportunity to rescue it before it got terminal.
I know that you know it Kryten
and Flying Farmer but it is also worth mentioning to less knowledgeable folk that too much oil pressure could cause as much damage as too little. Oil flow is more important in almost every case.And before you all start blowing steam out of your ears/arse/whatever, on my first engine I was fighting what I believed to be perpetual falling oil pressure. Cold start was invariably 45psi but this dropped to 20psi at hot idle, 2k + was 30-35psi. Full on panic mode ensued!
this was 25 years ago
I replaced the pump release valve which raised it a bit at hot idle but did nowt for the 2k+ rev range, by now terrified
I replaced the pump with a saab jobby which gave be 70psi+ from cold, but the oil pressure dropped to much the same when hot and at idle
BUT
A little knowledge here would have informed me of some of the behaviours displayed on the mechanical oil pressure gauge. i.e. coasting the oil pressure was ok, 55-60psi iirc, however hard acceleration and it dropped by upto 10psi
. this situation continued to get worse over the following 6 months. I tried to keep it below 3k rpm at all times
I believe that the main bearings were worn to such an extent that the 20/50 was squirting out the sides and dripping back into the sump, this was depriving other parts of the engine of oil, lower oil pressure may have avoided this situation and prevented the destruction of the block.
This happened suddenly, disassembly helped me to understand better what had happened.
Main bearings were all worn, centre was the worst by some margin. #2 and #4 were back to the copper. I believe that the crank vibration caused 2,3 and,4 main bearing caps to fracture. The engine ran but with a noticeable death knock from cold (this only happened once, and the last time that the engine ran) and oil pressure was no higher that 20psi. and barely 5psi at idle.
Not unusual for a Rover v8 to run at 10psi or less at idle, my first 3.9 Range Rover happily sat with the oil pressure light flickering at idle, far from ideal but it ran. Before you all start thumping the table and shouting "but what about the hydraulic chain tensioners?" Well I am not sure what pressure they need to run at but personally I never had rattling chains on the engine that died so can only assume that even when idle was recording below 10psi and the oil light was flickering the chains had enough oil pressure!! Kryten / flying farmer what are your feelings on chain tensioners and pressure required?
Rover v8 hydraulic tappets will work fine at very low pressure indeed
, I know that they are slightly different in that they are primed and tend to be very slow to leak back but this 3.9 v8 had done 180k miles so was hardly in the prime of yoof.
Now we all know about how wonderful hindsight is, so what would I do if faced with the same issue again?
Well first of all I wouldn't have fitted an oil pressure gauge without at least first understanding the characteristics and behaviours that it might display. After all without this all the gauge is tell you is that the gauge is working! Milothedog taught we that one
More importantly I think is to understand the design characteristics of the engine, what parameters was it designed to run under? Having had the luxury of building a few Stag v8 engines up from scratch and running them as if new I am no longer terrified if the cold start oil pressure is not reading 55psi, mine has been just below 50psi since I rebuilt it a few years back. it idles hot at around 30psi and runs somewhere around 40-45psi. Nothing ever changes, the consistency of it's behaviour is almost boring. I may as well remove the oil pressure gauge now because it should be used as a diagnostic device rather than something to distract while on the road.
So to you folk out there sobbing into your cornflakes because your stag v8 is running at whatever psi at idle? Rest thy sphincters. Your engine may well be past youth and it doesn't necessarily spell disaster. Key thing if you have an oil pressure gauge is less about what it displays and more about sudden changes in normal everyday behaviour. And if it does! try another gauge, preferably mechanical before loosing all hope and diving into the pit of despair.
Now that original engine of mine, was the unit fitted to the car when new
, the car had just under 100k miles on the clock. The cylinders were standard bore, needed a rebore but were serviceable. The crank journals were also standard, vandervell bearings and everything. The heads were unskimmed. Just wish that I had the opportunity to rescue it before it got terminal.